1. Field of the Invention
This invention relates to computer system input/output and, more particularly, to graphics transaction handling within an input/output node.
2. Description of the Related Art
In a typical computer system, one or more processors may communicate with input/output (I/O) devices over one or more buses. The I/O devices may be coupled to the processors through an I/O bridge which manages the transfer of information between a peripheral bus connected to the I/O devices and a shared bus connected to the processors. Additionally, the I/O bridge may manage the transfer of information between a system memory and the I/O devices or the system memory and the processors.
Unfortunately, many bus systems suffer from several drawbacks. For example, multiple devices attached to a bus may present a relatively large electrical capacitance to devices driving signals on the bus. In addition, the multiple attach points on a shared bus produce signal reflections at high signal frequencies which reduce signal integrity. As a result, signal frequencies on the bus are generally kept relatively low in order to maintain signal integrity at an acceptable level. The relatively low signal frequencies reduce signal bandwidth, limiting the performance of devices attached to the bus.
Lack of scalability to larger numbers of devices is another disadvantage of shared bus systems. The available bandwidth of a shared bus is substantially fixed (and may decrease if adding additional devices causes a reduction in signal frequencies upon the bus). Once the bandwidth requirements of the devices attached to the bus (either directly or indirectly) exceeds the available bandwidth of the bus, devices will frequently be stalled when attempting access to the bus, and overall performance of the computer system including the shared bus will most likely be reduced. An example of a shared bus used by I/O devices is a peripheral component interconnect (PCI) bus or an extended PCI (PCI-X) bus.
Many I/O bridging devices use a buffering mechanism to buffer a number of pending transactions from the PCI bus to a final destination bus. However buffering may introduce stalls on the PCI bus. Stalls may be caused when a series of transactions are buffered in a queue and awaiting transmission to a destination bus and a stall occurs on the destination bus, which stops forward progress. Then a transaction that will allow those waiting transactions to complete arrives at the queue and is stored behind the other transactions. To break the stall, the transactions in the queue must somehow be reordered to allow the newly arrived transaction to be transmitted ahead of the pending transactions. Thus, to prevent scenarios such as this, the PCI bus specification prescribes a set of reordering rules that govern the handling and ordering of PCI bus transactions.
To overcome some of the drawbacks of a shared bus, some computers systems may use packet-based communications between devices or nodes. In such systems, nodes may communicate with each other by exchanging packets of information. In general, a xe2x80x9cnodexe2x80x9d is a device which is capable of participating in transactions upon an interconnect. For example, the interconnect may be packet-based, and the node may be configured to receive and transmit packets. Generally speaking, a xe2x80x9cpacketxe2x80x9d is a communication between two nodes: an initiating or xe2x80x9csourcexe2x80x9d node which transmits the packet and a destination or xe2x80x9ctargetxe2x80x9d node which receives the packet. When a packet reaches the target node, the target node accepts the information conveyed by the packet and processes the information internally. A node located on a communication path between the source and target nodes may relay or forward the packet from the source node to the target node.
Additionally, there are systems that use a combination of packet-based communications and bus-based communications. For example, a system may connect to a PCI bus and a graphics bus such as an accelerated graphics port (AGP) bus. The PCI bus may be connected to a packet bus interface that may then translate PCI bus transactions into packet transactions for transmission on a packet bus. Likewise the graphics bus may be connected to an AGP interface that may translate AGP transactions into packet transactions. Each interface may communicate with a host bridge associated with one of the processors or in some cases to another peripheral device.
When PCI devices initiate the transactions, the packet-based transactions may be constrained by the same ordering rules as set forth in the PCI Local Bus specification. The same may be true for packet transactions destined for the PCI bus. These ordering rules are still observed in the packet-based transactions since transaction stalls that may occur at a packet bus interface may cause a deadlock at that packet bus interface. This deadlock may cause further stalls back into the packet bus fabric. In addition, AGP transactions may follow a set of transaction ordering rules to ensure proper delivery of graphics data. Thus, it is possible that some AGP response transactions may be delayed while waiting for other transaction types, such as PCI transactions, to be sent. Therefore, a peripheral interface circuit that may expedite handling of graphics transactions within an I/O node may be desirable.
Various embodiments of a peripheral interface circuit for handling graphics responses in an I/O node of a computer system are disclosed. In one embodiment, a peripheral interface circuit includes a buffer circuit coupled to receive packet commands. The buffer circuit includes a plurality of buffers each corresponding to a respective virtual channel of a plurality of virtual channels for storing selected packet commands that belong to the respective virtual channel. The peripheral interface circuit may determine whether a given one of the received packet commands is a graphics response belonging to a particular respective virtual channel. In response to determining that the given packet command is a graphics response belonging to the particular respective virtual channel, the buffer circuit may cause the given packet command to bypass the plurality of buffers.
In one specific implementation, the peripheral interface circuit includes a data buffer that may store data packets corresponding to the given packet command. The peripheral interface circuit may also include a bus interface circuit coupled to the buffer circuit and configured to initiate a bus cycle which corresponds to the given packet command and is suitable for transmission on a graphics bus.
In yet another specific implementation, the bus interface circuit may include a source bus and a target bus each configured to convey transactions between the graphics bus and the peripheral interface circuit. The source bus and said target bus may each include a command channel and a response channel. The command channel of the source bus may be configured to convey posted and non-posted commands initiated by a device on the graphics bus. The command channel of the target bus may be configured to convey posted and non-posted commands initiated by a device not on the graphics bus. The response channel of the source bus may be configured to convey response commands initiated by the device not on the graphics bus. The response channel of the target bus may be configured to convey response commands initiated by the device on the graphics bus.